fix architecture filtering on image selection

[edlerd]

Done

• fix architecture filtering on image selection, it needs to deal with non-aliased architectures
• adding test for architecture alias loading

Fixes #753

QA

1. Run the LXD-UI:
   • On the demo server via the link posted by @webteam-app below. This is only available for PRs created by collaborators of the repo. Ask @mas-who or @edlerd for access.
   • With a local copy of this branch, run as described here.
2. Perform the following QA steps:
   • browse images on instance create. Ensure the same architecture filters on the left are available as before.

[webteam-app]

Demo

Jenkins

demos.haus

[mas-who]

LGTM 👍