Add config for setting search_min_length capability

This adds the "search_min_length" setting to the frontend service which
allows to set the search_min_length capabilty which is e.g. used by
web.

Partial: #547
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f