Fix for #469 compose shows error now when length too long

[danschwarz]

No description provided.

[ihabunek]

@danschwarz I don't want to do this because we don't always detect max length correctly, and I don't want the user not to be able to send a longer message just because fail at this.

Besides, calculating max length is more involved than just len(content), beucause you need to take into account links which are shortened by the server.

If you look at https://mastodon.social/api/v1/instance you'll see characters_reserved_per_url:

{
  "configuration": {
    "statuses": {
      "max_characters": 500,
      "max_media_attachments": 4,
      "characters_reserved_per_url": 23
    }
}

A better fix would be to catch the exception when posting and display the error to the user.