Route-matching themes aren't respected on first load/refresh

[ybnd]

Describe the bug
While applying #1627 to a client repository I noticed that it broke route-matching themes in a way I didn't picked up on before:

• ThemeService.updateThemeOnRouteChange$ is no longer triggered when a page is first loaded, so the default theme gets used instead.
• Navigation is not affected

To Reproduce
Steps to reproduce the behavior:

1. Run a local DSpace instance with the following configuration:

     themes:
      - name: custom
        regex: search
      - name: dspace

2. Open the app at /home
   • It should use the dspace theme
3. Navigate to the /search page
   • It should use the custom theme
4. Refresh the /search page
   • It uses the dspace theme instead of the custom theme

Expected behavior
Route-matching themes should always be used on the matching routes.

Related work

• Improve initial page load #1627
• Dynamic theme improvements #1326

[ybnd]

@tdonohue this is a pretty major problem with #1627 that I should have caught, I think it's crucial that we merge a fix before 7.3

[tdonohue]

@ybnd : Feel free to work on it, I'll pull it over onto the 7.3 board. Bug fixes are always welcome, so if you can find a fix, please submit as soon as you can.