Improve performance of blob template dropdown rendering

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The following discussion from !7391 (merged) should be addressed:

• @nick.thomas started a discussion:

  Since we render all of these different template names on the same page, there's a performance optimization to be had from re-using the same TemplateFinder for all four methods.

  This implies that we'd make the type an optional restriction placed in params on the TemplateFinder, and merge the LicenseTemplateFinder into it somehow.

  The improvement is probably miniscule in CE, but could be really significant in EE - especially for a deeply-nested project with many applicable group templates. However, I expect the current behaviour to be performant for the common case (0 - 2 groups to take information from).

  I suggest we bump this to a performance technical debt follow-up issue and try to schedule it for %11.5 or %11.6 /cc @jramsay @DouweM