bugdown: Use CompiledPattern in AtomicLinkPattern.
bugdown: Use CompiledPattern in AtomicLinkPattern.

We avoid re-computing the regex string here, and we
also avoid re-compiling the regex itself.

I decided to put the "one_time" decorator in the
bugdown file itself, just to reduce friction in
folks reading the "buyer beware" comments.

Unfortunately, we can't use this for the
get_web_link_regex() function due to testing concerns,
so that continues to do an inelegant cache-with-global-var
scheme.

tommyip/zulip,showell/zulip,brainwane/zulip,eeshangarg/zulip,hackerkid/zulip,andersk/zulip,brainwane/zulip,timabbott/zulip,rishig/zulip,eeshangarg/zulip,kou/zulip,synicalsyntax/zulip,shubhamdhama/zulip,rht/zulip,andersk/zulip,brainwane/zulip,punchagan/zulip,synicalsyntax/zulip,rht/zulip,synicalsyntax/zulip,rishig/zulip,tommyip/zulip,eeshangarg/zulip,synicalsyntax/zulip,shubhamdhama/zulip,showell/zulip,zulip/zulip,punchagan/zulip,kou/zulip,zulip/zulip,kou/zulip,brainwane/zulip,rishig/zulip,hackerkid/zulip,eeshangarg/zulip,zulip/zulip,shubhamdhama/zulip,timabbott/zulip,rht/zulip,andersk/zulip,kou/zulip,eeshangarg/zulip,showell/zulip,shubhamdhama/zulip,eeshangarg/zulip,zulip/zulip,synicalsyntax/zulip,eeshangarg/zulip,timabbott/zulip,punchagan/zulip,tommyip/zulip,hackerkid/zulip,showell/zulip,showell/zulip,synicalsyntax/zulip,tommyip/zulip,andersk/zulip,andersk/zulip,timabbott/zulip,timabbott/zulip,timabbott/zulip,hackerkid/zulip,rishig/zulip,rht/zulip,hackerkid/zulip,brainwane/zulip,punchagan/zulip,zulip/zulip,shubhamdhama/zulip,zulip/zulip,hackerkid/zulip,rishig/zulip,hackerkid/zulip,rht/zulip,rishig/zulip,tommyip/zulip,kou/zulip,shubhamdhama/zulip,andersk/zulip,timabbott/zulip,tommyip/zulip,showell/zulip,brainwane/zulip,synicalsyntax/zulip,punchagan/zulip,zulip/zulip,punchagan/zulip,rht/zulip,kou/zulip,andersk/zulip,rht/zulip,showell/zulip,rishig/zulip,shubhamdhama/zulip,kou/zulip,brainwane/zulip,tommyip/zulip,punchagan/zulip