fixed two bugs when switching accounts from the ComposeView

[dvorakroth]

Summary

fixes issue #72

relevant quotes from there that pretty much sum it up:

Multiple accounts with different maximal chars available. When switching accounts while writing a post: available character count is not updated to the newly selected account.

and

if you tap your profile pic a second time to switch back to the original account, it won't show it in the menu

i fixed the first part by:

• making CompositionViewModel.maxCharacters into a Publisher
• making the character count recalculate when $maxCharacters is updated
• updating maxCharacters via a helper method whenever NewStatusViewModel.$identityContext is updated

and i fixed the second part, in CompositionView, by moving the call to parentViewModel.identityContext.$authenticatedOtherIdentities.sink() into the trailing end of the callback given to parentViewModel.$identityContext.sink().

the problem there, originally arose because whenever the user switches accounts, the identityContext gets completely replaced by a new object. so when, later on, eventually, the new identityContext.authenticatedOtherIdentities got updated, nothing was subscribed to it. and so calling identityContext.$authenticatedOtherIdentites.sink() on every change to identityContext has fixed it

Other Information

• I have signed Metabolist's Contributor License Agreement