Switch to Regexp::Assemble for whitelists and banned file lists

[rjl]

We can improve the efficiency of the regular expression lists we use for whitelists/blacklists and for per-user banned file attachment lists by precompiling all of the regular expressions for a given user into a single regex that tests much faster than hundreds of individual regexes. The ​Regex::Assemble module does this. This feature should be considered a prerequisite for #48.