Fix static files links for release build

[will-moore]

Fixes the failure to load static files for the 8.0.1.rc1 release (when deployed on outreach server).
This was the first test of installing a release build (different from the local built that is tested by merge-ci).

UPDATE: problem was due to base url being set when we build via gh-actions (used for gh-pages deploy) but gh-actions is also used for release build.

Now, we ONLY set base url when we specifically build for gh-pages.

I have pushed this branch to my https://github.com/will-moore/omero-figure/tree/master branch so that gh-pages is tested to deploy to https://will-moore.github.io/omero-figure/

To test: gh-pages should still deploy correctly (app works etc) and other deploys work too!

[pwalczysko]

Flake8

flake8.main.application   MainProcess    659 INFO     Found a total of 47 violations and reported 3
./omero_figure/views.py:160:80: E501 line too long (86 > 79 characters)
./omero_figure/views.py:161:80: E501 line too long (86 > 79 characters)
./omero_figure/views.py:162:80: E501 line too long (119 > 79 characters)

[pwalczysko]

I can confirm that when I did the addition from this PR onto my views.py inside the RC of omero-figre installed on a testing VM RHEL9, the problem with the css on opening the figure app is solved.

I still have a problem with "Open With" though... it does not place the images selected into the figure....

[pwalczysko]

1. Deployed on ghpages https://pwalczysko.github.io/omero-figure/. Works as expected.
2. Deployed using npm dev run as per README. Works as expected.
3. I think for the RC workflow we need a new RC.