Allow to specify alternative text for uploads to Markdown fields

Problem to solve

When attaching a file to an issue description or comment, the alternative text is automatically filled in from the file name. This typically leads to text such as Screenshot_2019-02-26_at_12.54.42 rather than a proper description of the image or video. This is not inclusive and also not helpful in case an image or video cannot be loaded.

Target audience

Basically everybody...

Proposal

Ask for alternative text for upload prior to adding them to the Markdown field.

Documentation

What does success look like, and how can we measure that?

We have better alternative texts for uploads in GitLab.