[Feature] Make downloading an atlas robust

While an atlas is being downloaded, double-clicking on said atlas should not break anything. I'm not sure it does currently, but I think it might?

**Describe the solution you'd like**
Full robustness and good UI for this. Maybe disabling and greying out the row of the atlas currently being download. Related to #62

**Describe alternatives you've considered**
None yet, but open to suggestions

**Additional context**
Continuation of #7

No files or tests are named. Start by locating the atlas download entry point and the UI code for atlas rows, then reproduce a double-click during an active download. Done means repeated activation does not disrupt the download and the active row provides clear disabled or in-progress feedback.

Written by the indexing model from the issue text.