What kind of change does this PR introduce?

Bug Fix (according to the issue)

Why was this change needed?

Fixing max length setting for Mastodon posts.

It is impossible to write a post longer than 500 characters regardless of the Mastodon instance’s limit. (...) Postiz uses a hardcoded 500-character limit.

Fixes #1083

Checklist:

Put a "X" in the boxes below to indicate you have followed the checklist;

• I have read the CONTRIBUTING guide.
• I checked that there were not similar issues or PRs already open for this.
• This PR fixes just ONE issue (do not include multiple issues or types of change in the same PR) For example, don't try and fix a UI issue and include new dependencies in the same PR.