Add loading notification to metis and gnomon ui

[dtm2451]

For folders with LOTS of files, it can be a bit scary to see the current "No Content" label for 5+ seconds while the actual contents are just loading. PR adds an icon + "Loading" text while the contents are being fetched.

For gnomon rule-editing, it is currently possible to save changes that start from a blanked defaults setup. Showing a loading icon, instead of the regular components filled in with the fake default state, is simple enough and would remove that path!

ToDo:

• [x] make the icon spin
• [x] add a delay so the icon only shows if loading takes more than ~100ms and doesn't blip in for every single folder.
• [x] implement in gnomon rule-editor too! (simpler: no delayed 'showLoading' needed because there aren't multiple pages (metis: nested folders) that a user would need to click through here)

[dtm2451]

Testing:

• You can test by setting a networking latency in your browser dev tools.
• gnomon side:
  1. Navigate to the gnomon rule-editor page for a project that you have set rules for and ensure that it 1) gives a spinning icon + "Loading" text during the latency period, then 2) shows your actual rules afterwards.
  2. Navigate to the gnomon rule-editor page for a new project or one without rules set up, and ensure that is also 1) gives a spinning icon + "Loading" text during the latency period, then 2) still shows the editable default setup afterwards.
• metis side:
  1. Navigate into any bucket as the change is only for folders.
  2. You should see a spinning icon + "Loading" text underneath the "No Contents" display during the latency period.