fix(toolbar): Fix icon padding for ripples, and vertical alignment in FF/IE/Edge

[kfranqueiro]

• Change align-items to center for toolbar section element to fix vertical alignment in IE/Edge and also mitigate the adjustments to vertical padding in this PR
• Adjust vertical padding on toolbar items so that ripples applied to icons don't extend further than they should
• Add margin instead of increasing padding on leading and last toolbar icons, again to avoid excessive ripple size

This should not cause a visible difference in icon padding/arrangement (other than vertically aligning it properly in IE/Edge).

Fixes #1067 (finally)
Also fixes #1296 related to vertical alignment (I re-checked and indeed it's not just IE/Edge, it affects FF too, huh.)

[lynnmercier]

Should we call @include mdc-states(text-primary-on-primary) from within the CSS for mdc-toolbar__icon? And then remove this mdc-ripple-surface from the demo?

[kfranqueiro]

I can probably add ripple styles to mdc-toolbar__icon if it's safe to assume all toolbar icons are intended to be interactable (which I think is the case judging by the spec).

If I do that, I should probably also add logic to toolbar's JS to instantiate ripples for its icons.

[lynnmercier]

I think we should do that.

[kfranqueiro]

I just realized the fix this branch includes for #1296 (and also to support the main fix for #1067) actually causes problems for flexible toolbar, so I might need to find a different way to fix the vertical alignment...

[kfranqueiro]

@williamernest was cool enough to find a resolution to the snag I ran into with flexible toolbar, and I've committed the rest of my stuff on top of that. Will's now hammering out one remaining issue but then I think this is ready for review again.

[codecov-io]

Codecov Report

Merging #2138 into master will decrease coverage by 0.17%.
The diff coverage is 22.22%.

@@            Coverage Diff             @@
##           master    #2138      +/-   ##
==========================================
- Coverage   99.21%   99.04%   -0.18%     
==========================================
  Files          99       99              
  Lines        3967     3976       +9     
  Branches      510      510              
==========================================
+ Hits         3936     3938       +2     
- Misses         31       38       +7

Impacted Files	Coverage Δ
packages/mdc-toolbar/constants.js	100% <ø> (ø)	⬆️
packages/mdc-toolbar/index.js	82.5% <22.22%> (-17.5%)	⬇️

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