This change addresses some problems inherent to the way the formatting

toolbar was placed in CodeMirror's DOM, and uses the opportunity to turn

this all into a documented component and tweak styling.
TextEditorView is now backed by a .rb-c-text-editor CSS component

class, replacing the old .text-editor and plain textarea /

.CodeMirror nesting. This gives us a solid structure for the

component that we can build upon. Some scattered rules were cleaned up

in the process, helping reduce how widespread our styling was.
The formatting toolbar now lives within .rbc-text-editor, and not

within .CodeMirror. This addresses problems with spacing and layout.

By being placed in .CodeMirror, we ended up overlapping a horizontal

scrollbar, and this ends up causing some layout wonkiness when stressing

the text editor (such as filling the comment dialog with enough text to

force scrolling).
To provide some visual separation, the background of the toolbar is now

a light grey, with 2em of spacing above it (technically, an offset of

2em at the bottom of the CodeMirror scroll container). This gives enough

breathing room to write without the toolbar being too far away.
In the inline editors with growing text fields, we get those 2em of

space at all times. In the comment dialog, it's allowed to reach down to

the top of the toolbar, so that the scrollbar is flush against it. This

mirrors the behaviors prior to adding the toolbar.
To reduce visual noise and focus on the buttons, the vertical separators

in the toolbar have been replaced with additional spacing. The hit areas

for the buttons are also a bit larger now, making them easier to press

on mobile.

Tested the following in Chrome, Safari, and Firefox with both plain text

and Markdown:

Inline editors in the review request field
Inline editors in the review dialog
Comment dialog

Each was tested with small amounts of text and many pages of text, to check

for any scrolling or padding issues.
Made sure that the ancient styles that were removed weren't actually being

used and that their removal didn't affect any display or usage.