Get character limit from instance metadata

[lanodan]

Pleroma and Mastodon with https://github.com/tootsuite/mastodon/pull/5697 (included in glitch-soc) gives the max_toot_chars key in /api/v1/instance. Could you use this value to change the character limit? (which is currently hidden in dconf settings).

[bleakgrey]

Sure

[cleac]

I've made a proposal of changes that implement this feature. I am aware that it won't work with mastodon as far as mentioned pull request had been rejected, but this must be pretty great feature for any other mastodon-api-compatible services (like pleroma).

Would be happy if somebody reviewed it :smile: