fix: add alt text support to Mastodon media uploads

[sginji]

What kind of change does this PR introduce?

Adds support for alt text in Mastodon media uploads by passing the description parameter to Mastodon's API

Why was this change needed?

#948

Other information:

Not yet

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.

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