Prevent false-positives of the rich embed filter

https://github.com/python-discord/bot/issues/293

The rich embed filter is plagued by false positives now Discord has
added more custom preview embeds for various websites. Since these
embeds have the `rich` type instead of the `link` type, these embeds
triggered the filter we had in place.

This commit remedies that by using the existing URL regex pattern to
list all the URLs contained in the message content and then checking
if the embed url is a member of that list. If so, it's very likely
that the embed was auto-generated from that URL, so we should ignore
it. This approach deviates slightly from that outlined in #293.

This does increase the probability of a false-negative, as a "true"
user-generated rich embed could also have a url that's contained in
the message body. However, I've checked most of the triggers we have
had in the past and none of the legitimate triggers would have been a
false-negative under the new rules. Therefore, I think it's very
reasonable to adopt this strategy.

In addition to the change in behavior of the rich embed filter, I
have also kaizened the existing regex patterns by compiling them at
load time. Since we check a lot of regex patterns for every message
received by the bot, this should be beneficial for performance.

0 files changed, 0 insertions, 0 deletions