Fixes and tweaks for Gravatar use

[Deadpikle]

This PR rolls several small fixes/tweaks into one PR, all related to Gravatar use:

• Fixed the text color in EditableTextModeItem in dark mode (the email input for Gravatar was dark text on dark background in dark mode)
• Fixed Gravatar email input allowing more than one line (adds maximumNumberOfLines toEditableTextModelItem, implemented via/controlled by delegate method and setting textView.textContainer.maximumNumberOfLines)
• Added SHA-256 string hashing extension and used this for Gravatar hashes as per documentation here
• Allow manual clearing of image cache as debug step if avatar loading isn't working. This...honestly I'm not sure is needed. However, I am facing an interesting problem where the simulator and a newly built app on physical device loads custom avatars OK, and yet the TestFlight version doesn't. I'm wondering if my poor internet here caused the default image to be cached somehow, and thought I'd try this as a first step to testing for a fix. (See #635 -- I can get this user's Gravatar to load successfully in the app using the new custom email feature, so I'm not sure why there are issues for them or for me.)
  • I have tried to replicate this error and cannot replicate it reliably. I'm wondering if perhaps there is a delay of some kind on the Gravatar end, the user's avatar isn't found, and then the default value is cached. Then the user can't clear the cache to try again. This is also why I am attempting an update to using SHA-256 hashes in case the current MD5 method has any issues. Sort of shooting in the dark, here. Open to ideas on how to debug this one.

[Deadpikle]

Removed the MD5 files as you're correct, that was the only place it was being used.

I doubt clearing the cache will help since the different URLs should be different cache keys, but I guess it's worth a try.

Yeah, sort of a shot in the dark if things are still not working with the SHA256 change for new URLs. Went ahead and added a debug line and I can hook up my phone into my Mac when the new TestFlight build goes out to test.

Rebased on latest master.