Fix file description labels to use "alt text" terminology

What does this MR do?

Sharkey currently uses "Caption," "Description," and "Alt text" to refer to the ~same thing. These phrases have different meanings in the context of posting content online, and the mismatch has caused confusion for me and other users on my Sharkey instance. This MR unifies all labels in Sharkey that concern file descriptions to use the phrase "alt text." This will address #938 (closed).

Here is what each UI element looks like with these changes applied:

A couple of lingering questions, which may or may not warrant further discussion:

• In a user's Drive, would we like to keep the "Description" label, and perhaps add an additional note e.g. "(Used as alt-text when posting content)"? For now, I've just made everything say "Alt text" or similar.
• I couldn't figure out if sharkey-locales/en-US.yml has an intentional order. I am happy to move things around as necessary.
• Can/should this change be made for other languages?

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