coretasks: alert owner if nick in RPL_WELCOME does not match config

[dgw]

Description

Inspired by a recent PR to the "horse docs" site (ircdocs/modern-irc#146), which adds a paragraph about this.

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches

New log output

The result of "test[ing] the functionality of the things this change touches".

stdout/logging

...
[2021-11-27 20:43:25,272] sopel.coretasks      INFO     - End of client capability negotiation requests.
[2021-11-27 20:43:25,307] sopel.coretasks      CRITICAL - RPL_WELCOME indicated the server did not accept the bot's
                                                          configured nickname. Requested
                                                          'ThisIsAVeryLongNickThatRizonWillTruncateBecauseItsLongerThan30Characters';
                                                          got 'ThisIsAVeryLongNickThatRizonWi'. This can cause unexpected
                                                          behavior. Please modify the configuration and restart the bot.
[2021-11-27 20:43:25,310] sopel.coretasks      INFO     - Joining 1 channels (with JOIN throttle OFF); this may take a moment.
...

raw log

...
2021-11-27 20:43:25,307 <<      ':irc.sxci.net 001 ThisIsAVeryLongNickThatRizonWi :Welcome to the Rizon Internet Relay Chat Network ThisIsAVeryLongNickThatRizonWi\r\n'
2021-11-27 20:43:25,309 >>      "PRIVMSG dgw :Hi, I'm your bot, ThisIsAVeryLongNickThatRizonWillTruncateBecauseItsLongerThan30Characters. The IRC server didn't assign me the nick you configured. This can cause problems for me, and make me do weird things. You'll probably want to stop me, figure out why my nick isn't acceptable, and fix that before starting me again.\r\n"
2021-11-27 20:43:25,310 >>      'MODE ThisIsAVeryLongNickThatRizonWillTruncateBecauseItsLongerThan30Characters +B\r\n'
...