Better Highlights: Fix Wrong Color for Migrated Phrases (#1568)

* Better Highlights: Fix wrong color for migrated phrases

Prior to this commit, no default color was set when an "old" highlight
phrase (one added prior to #1320 / 5957551) was deserialized. This
commit makes highlight phrases uses the default self-highlight color for
these situations. This approach is reasonably sensible since that color
is also similar to the old highlight color.

Fixes #1565.

* Update default self-highlight color

The new default color was suggested in [1] by @RAnders00. Refer to the
link for further information.

[1]: https://github.com/Chatterino/chatterino2/issues/1565#issuecomment-590441625

* Theme: Remove highlight color

As highlight color is independent of the selected theme now, the member
has been removed from the `Theme` singleton. Instead, the fallback theme
color is defined in `HighlightPhrase` now.

Uses of `themes->messages.backgrounds.highlighted` have been replaced
with `HighlightPhrase::FALLBACK_COLOR` accordingly.

* Update src/controllers/highlights/HighlightPhrase.hpp

attempt to just remove constexpr

Co-Authored-By: Ruben Anders <ruben.anders@robotty.de>

* Initialize FALLBACK_COLOR outside header file

* Rename FALLBACK_COLOR to FALLBACK_HIGHLIGHT_COLOR

In preparation for the next commit introducing a new variable.

* Moved subscription highlight color into HighlightPhrase

* Use actual subscription color as a fallback

Co-authored-by: pajlada <rasmus.karlsson@pajlada.com>
Co-authored-by: Ruben Anders <ruben.anders@robotty.de>

src/controllers/highlights/HighlightPhrase.cpp

@@ -2,6 +2,9 @@
 
 namespace chatterino {
 
+QColor HighlightPhrase::FALLBACK_HIGHLIGHT_COLOR = QColor(127, 63, 73, 127);
+QColor HighlightPhrase::FALLBACK_SUB_COLOR = QColor(196, 102, 255, 100);
+
 bool HighlightPhrase::operator==(const HighlightPhrase &other) const
 {
     return std::tie(this->pattern_, this->hasSound_, this->hasAlert_,