Make themes structured data instead of read from file

[natefoo]

This way the role does not modify files in the playbook, and moves in line with other recent changes to move more of Galaxy's extraneous configs into structured data.

@mira-miracoli if you approve I'll update the documentation.

[mira-miracoli]

cool, less files!
But the directory structure for the subdomain static files has to stay I guess?

[natefoo]

Yes I think so since the same directory structure is used for the subdomain static files themselves.

[natefoo]

In working on the documentation and defaults changes I noticed, I don't think we need the "default base domain" entry on the *_base_host config vars. For example, if you have:

themes_config_file: themes_conf.yml
themes_config_file_by_host:
  usegalaxy.eu: themes_conf.yml
  assembly.usegalaxy.eu: assembly.yml

Then the usegalaxy.eu entry is redundant since, if there is no matching hostname for usegalaxy.eu found in themes_config_file_by_host, it would fall back to the value of themes_config_file. So I removed this bit, but if I have this wrong and it doesn't work like I think it does, let me know.

[mira-miracoli]

I think it did not work without, but we can try :)

[natefoo]

You're right, I have some additional hostnames set up for usegalaxy.org to reach my individual VMs directly (e.g. https://galaxy-web-05.galaxyproject.org/) and the themes aren't available via that route.