Investigate Generating ExpressSearchConfig

[acwhite211]

Investigate generating the ExpressSearchConfig file dynamically when the /context/available_related_searches.json API is called. This is to restore behavior prior to v7.10. Replace the use of using a default ExpressSearchConfig.xml in the static files.

Described in detail from this comment #6290 (review).

Approving this, but with three important notes:

1. The testing checklist here does not verify that behavior has not been modified between 7.9.6.2 and 7.10, which may leave us open to issues if we inadvertently break this behavior.

2. Someone in @specify/developers  should open a new issue to fix the fact that this (`available_related_searches.json`) file is not being automatically generated.
   https://github.com/specify/specify7/blob/production/specifyweb/frontend/js_src/lib/tests/ajax/static/context/available_related_searches.json
   My understanding is that before 7.10, Specify would automatically create this file based on the list of available express search options. See below:
   https://github.com/specify/specify7/blob/56bebce77488a13783e9d43782330d348a7f5896/specifyweb/context/views.py#L412-L425
   
   These 'related searches' are defined in this file:
   https://github.com/specify/specify7/blob/56bebce77488a13783e9d43782330d348a7f5896/specifyweb/express_search/related_searches.py#L25-L37
   
   This means it would be theoretically quite simple to extend the default simple search behavior using the old approach, but with this patch (does not fix the underlying issue, simply provides a default static file), it adds another level to the complication.

3. Before merging, can you explain how was this default `expresssearchconfig.xml` sourced? I'm concerned it may change the default searching behavior in the app.
   Upon removing all `ExpressSearchConfig` app resources in a database and logging in through Specify 6, I get this which seems to be as "default" as it comes:
   [DefaultExpressSearchConfig.txt](https://github.com/user-attachments/files/19054962/DefaultExpressSearchConfig.txt)
   The file in this PR is nearly [twice the size and is structured differently](https://raw.githubusercontent.com/specify/specify7/issue-5470/config/common/expresssearchconfig.xml).