implement release channels

[f321x]

Implements release channels so the user can select either a stable or beta release channel in the SettingsDialog for update notifications.
Adapts the release_www.sh and make_download scripts to handle alpha and beta versions which are detected by an 'a' or 'b' in the version name.
The version fields are added into the extradata field of the version file as version_alpha and version_beta and the extradata field is being signed too in the extradata_hash_signature field.
The update checker is supposed to return the newest version of those contained in the version file which is
allowed according to the user configured option.

[SomberNight]

I started looking into this but it is turning into quite a rabbit hole...
The root cause is here:

electrum/run_electrum

Lines 247 to 249 in 9b24316

	# decode json arguments
	if cmdname not in ('setconfig',):
	args = list(map(json_decode, args))

I would say it's a bug that e.g. $ ./run_electrum -v --testnet -w ~/.electrum/testnet/wallets/9dk -o signmessage tb1qq2tmmcngng78nllq2pvrkchcdukemtj5s6l0zu "5" does not work, due to the message getting json-decoded to an int.
Anyway, I am still looking into this.

[ecdsa]

maybe these 3 lines can be deleted? if something needs to be converted to json, it will be done in parser.parse_args. Example: The following command works after deleting these 3 lines

@@ -256,9 +256,6 @@ async def run_offline_command(config: 'SimpleConfig', config_options: dict, wall
         print_msg("Warning: running command offline")
     # arguments passed to function
     args = [config.get(x) for x in cmd.params]
-    # decode json arguments
-    if cmdname not in ('setconfig',):
-        args = list(map(json_decode, args))
     # options
     kwargs = {}
     for x in cmd.options:

/run_electrum --testnet -o serialize '{                                                                                                                                                     
            "inputs": [                                                                                                                                                                        
                {"prevout_hash": "9d221a69ca3997cbeaf5624d723e7dc5f829b1023078c177d37bdae95f37c539", "prevout_n": 1,                                                                           
                 "value_sats": 1000000, "privkey": "p2wpkh:cVDXzzQg6RoCTfiKpe8MBvmm5d5cJc6JLuFApsFDKwWa6F5TVHpD"}                                                                              
            ],                                                                                                                                                                                 
            "outputs": [                                                                                                                                                                       
                {"address": "tb1q4s8z6g5jqzllkgt8a4har94wl8tg0k9m8kv5zd", "value_sats": 990000}                                                                                                
            ]                                                                                                                                                                                  
        }'

[ecdsa]

ah, I forgot that this will not work with curl/jsonrpc

[SomberNight]

yes, it's not so simple. (1) jsonrpc vs (2) online CLI vs (3) offline CLI all work very subtly different. We need explicit unit tests, as otherwise the whole thing is extremely fragile.
I have some WIP changes from when I last commented above:
SomberNight@ef57336