Improvements to Message Search (#1237)

* Ran clang-format

* Implement user-specific search in message history

This functionality was originally requested in #1236.

This commit changes the SearchPopup::performSearch method so that only
messages from specific users can be shown.

In order to filter for a specific user, enter their username with a
leading '@' in the search popup. You can also add an additional search
phrase which will also be considered in the search.

* Naive implementation for "from:" tags

Rebase later?

* Cleverer (?) version using Predicates

Commit adds two POC predicates: one for the author of messages, and one
for substring search in messages.

Problems/TODOs:
* Best way to register new predicates?
* Clean up tags (e.g. "from:") or not?
* Test combinations of different predicates

* Add a predicate to check for links in messages

* Remove a dumb TODO

* Rewrite SearchPopup::performSearch to be cleaner

* Ran clang-format on all files

* Remove TODO I missed earlier

* Forgot to run clang-format

peepoSadDank

* Re-use {}-initialization

Was accidentally removed when fixing earlier merge conflict.

* Does this fix line endings?

No diffs are shown locally, hopefully Git doesn't lie to me.

* Rename "predicates" directory to "search"

Resolving one conversation in the review of #1237.

* Use LinkParser in LinkPredicate

Resolving a conversation in the review of #1237.

* Predicates: Use unique_ptr instead of shared_ptr

Resolves a conversation in the review of #1237.

* Refactor of SearchPopup and AuthorPredicate

Resolving some points from the review in #1237.

* Moved parsing of comma-seperated values into AuthorPredicate
  constructor.
* Rewrite SearchPopup::parsePredicates as suggested.
* Deleted now redundant methods in SearchPopup.

* MessagePredicate::appliesTo now takes a Message&

... instead of a MessagePtr.

This resolves a conversation in the review of #1237.

* Run clang-format on two files I missed

* AuthorPredicate: Check for displayName & loginName

Resolving conversation on #1237.

src/messages/search/SubstringPredicate.cpp

@@ -0,0 +1,15 @@
+#include "messages/search/SubstringPredicate.hpp"
+
+namespace chatterino {
+
+SubstringPredicate::SubstringPredicate(const QString &search)
+    : search_(search)
+{
+}
+
+bool SubstringPredicate::appliesTo(const Message &message)
+{
+    return message.messageText.contains(this->search_, Qt::CaseInsensitive);
+}
+
+}  // namespace chatterino