check .htaccess effectiveness

[codeling]

As an administrator, I want to see whether the .htaccess file can be used for blocking. The .htaccess option gets grayed out if there is anything preventing it from being used, and an informative message is shown as to why this occurs.
This includes:

• server type is not apache
• .htaccess is not read- or writable by apache user

[codeling]

A basic check of this is added via codeling/com_bfstop@8b6278f

In a future version, this should also affect whether .htaccess blocking is even allowed to be enabled (see also #88).