Allow downloading just the stitched image, and don't zip DZIs

Most users are only interested in the stitched image, and will only look at the raw images if there's either a problem or an interesting feature in the stitch. Either way, downloading the JPEG alone will be much faster than downloading all images, which after !251 (merged) can run into the gigabytes.

Also do not zip DZI images or their tiles

This MR contains the following

• A button in the scan tab to download JPEG, which is greyed out if there's no stitched image (yet)
• A new scan endpoint to download a stitched image from a scan, based on the scan name
• Creates a new Vue component EndpointButton for handling downloading not from actions
• Bonus rename every variable called path in smart scan to specify if it's a folder path, file path, scan path... Removed as this conflicted with !294 (merged)
• Do not zip DZI images or their tiles

In a follow up issue, improve the user interface - one button, with a default to download stitched JPEG, but the dropdown option for other combinations of files

edited to:

The slightly URL for the endpoint is /smart_scan/get_stitch/<scan-name>.

The changes to CSS close #441 (closed)

Merge checklist:

• All new/changed functions have up to date typehints and docstrings
• Any UI updates have a before and after screenshot in the MR description
• Unit testing of new functionality has similar or better coverage that similar existing code
• This code has been tested on a microscope
  • With webapp has been recompiled (if changed)
  • With updated settings JSON (if changed)
  • SmartScan and live stitching work