Allow Control over Hard vs. Soft Rewrite Flush

[ikailo]

Many times when the rewrite rules are flushed, it really only requires that the rules be written to the database and not .htaccess. In some cases, re-writing .htaccess can cause detrimental changes to the mod rewrite rules.

This patch allows a plugin author to have control over whether a hard flush is executed when flush_rewrite_rules() is called.

Introduces filter: flush_rewrite_rules_hard

Depends on #14546

Any filter here should be in the method directly, not the function wrapper.

Moved filter to method