Improvement: Respect branded and legacy activity icons when changing the activity icon purpose color

[abias]

Not all activity plugins have monologo icons which can be nicely tinted with Boost Union's "Activity icon purposes" feature.

There may be activities like BigBlueButton which ship with a multicolored brand icon (as the brand appearance should be preserved) or which still ship with a legacy activity icon only.

In both cases, changing the activity purpose in Boost Union leaves just a white / weird icon.

On Moodle 4.3:

On Moodle 4.4:

On the other hand, in Moodle 4.2 / 4.3 activities icons can have a nofilter class and in Moodle 4.4 they can have a isbranded class which indicate that the icon should be left as it is.

Most of the time fixing the icon appearance with custom CSS is straightforward.
However, we should investigate more real life edge cases and then, if possible, enhance Boost Union in a way that these white / weird icon effects are mitigated.