Pill buttons consistency

[Grizzly127]

Button design changes concerned mainly the pill buttons.
Now they all have consistent features:

• filled icons
• icon size: 24px
• font-size: 14px

Except for the icons at the Multiselection bar. They are still too small, but I can't find the place to change them.

[Grizzly127]

@vmalieske will change the icons when working on the rest of the multi-selection issues.

[vmalieske]

Icon adjustments of multiselection bar will be implemented with issue #177 (the other multiselection adjustments).

[HeyItsBATMAN]

I added a commit re-factoring the fixes applied by this PR.
Generally, everything was fine with the PR, but having to redefine the sizes for all mat-icons in a lot of different style definitions will make it harder down the line to keep them aligned.
I moved the font-size, width and height settings to the stylesheet defining the Material Symbols font, and applied the classes on the remaining <mat-icon>-elements instead.

I will open up a new issue on further simplification measures we can apply, but still merge this.