Fix mastodon alt text

[tcarreira]

What kind of change does this PR introduce?

Bug Fix or Feature, depends on the perspective 😃

Why was this change needed?

Fixing alt text for Mastodon posts

When adding alt text to the image, the Mastodon Provider does not seem to include it when posted. Image still shows as "No Alt Text", which may hurt its spread in certain communities.

Fixes #948

Tested on my postiz server. It works on my machine 😁

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.

[vercel]

@tcarreira is attempting to deploy a commit to the Listinai Team on Vercel.

A member of the Team first needs to authorize it.