Markdown and common toolbar icons consistency pass

[chryw]

#50438
@misolori @bpasero
I did a consistency pass to some icons on the editor top right toolbar area.

• visually aligned these icons
• aligned svg code style (responsive, hide halo and foreground, etc.)
• removed some unnecessary css styles so they don't override the global size of these icons

There might be other icons could show up in that area. I only tested below icons and made sure they look good. Please let me know if this change makes other icons inconsistent.

[bpasero]

@chryw the ellipsis icon seems a bit inconsistent now:

Search (new left, old right, seems to be smaller now):

Panel (new left, old right, seems to be much larger now):

[bpasero]

ping @chryw @misolori

[chryw]

@bpasero will fix today

[bpasero]

@chryw thanks, I merged this to master for our June release 👍