Allow extensions to modify the minimum search length in the Search component

[dsevillamartin]

Feature Request

Is your feature request related to a problem? Please describe.
Allow modifying the minimum search length for the Search component.

Describe the solution you'd like
Use this.constructor instead of Search directly so that subclasses can modify the static MIN_SEARCH_LEN

Justify why this feature belongs in Flarum's core, rather than in a third-party extension
This is core code that would allow extensions to not have to copy & paste the search handling to modify this aspect.