Default mail client startup prompt

[freaktechnik]

• This is more of a heads up and I'm not sure what exact action might be needed or not. In Bug 2030856 - Thunderbird does not check if it is the default mail client at start-up we discovered that we haven't been prompting if TB is no longer the default mail client at start up since we moved the account setup to a tab... As such, I wouldn't be surprised if a lot of people got prompted when they'll get the next ESR or even when they get the next release that includes the fix. I'm expecting to land the fix for startup in 151.
• we haven't checked if we're the default at startup since 91 and ever since then we only showed the prompt after a mail account setup. <-- both of these things will tentatively land in 151
  [roland edit]: Moved 2024725 aka `net.thunderbird:// to a separate github issue, MODIFY articles about OAuth since we are moving to net.thunderbird:// #188:
  I think we'll also have a big round of prompts with Bug 2024725 - add a custom scheme application handler for OAuth redirects.

CC @darktrojan