Now that existing message specs each have a `canBeSidebarSource` method, I need to replace instances of `isInvalidSidebarSource` (besides in unit tests) with `messageSpec.canBeSidebarSource`.
This covers most of the replacements, there are two replacements in `message-fetchers.js` that I'm runnning into flow issues with (see [[
https://linear.app/comm/issue/ENG-4849/thread-from-robotext-about-pinned-message-causing-errors#comment-5fbd94f3 | here ]] for context tracking this, so I'll put up a diff later in the stack that addresses those instances.
Addresses Part 1c of [[ https://linear.app/comm/issue/ENG-4849/thread-from-robotext-about-pinned-message-causing-errors | ENG-4849 ]]
Depends on D9325