* fix(welcome-page): css tweaks in prep for branded welcome page

- Watermarks should no longer depend on toolbar size. The left watermark made
  room for the toolbar when the toolbar was on the left side of the screen, but
  the toolbar has been moved to the bottom. The right watermark...well it'll
  clash with the vertical filmstrip but at least the margins will be consistent
  with the left watermark.
- Apply new font-family so fonts are more likely to be consistent across the
  app. Design likes SF UI and keeps requesting it so use it by default.
- Change sizings of welcome page header to be more responsive. This will help
  the header be scrollable when there is no additional content and the header
  overflows.
- Change colors of the welcome page header and remove background image that
  was in the header. Leave in the dom for the background image in case other
  deployments need to continue showing an image.
- Add a period to the title of the welcome page.
- Move watermarks dom location as it is not part of the header; it's part of the
  whole page.

* [squash] Size and font adjustments. Renaming.

This is needed for newer versions of electron that might
use the iframe integration of jitsi-meet. Newer versions
seem to have some kind of regression with setting height
and width to 100%.

I'm unsure why all textareas need overflow hidden. Doing so
essentially overrides what I would expect to be standard textarea
behavior. I would rather remove the reset and fix any areas that
have issues.

Safari will prevent proper input behavior when user-select none
is applied. It prevents such actions as putting in a room lock
password or setting a room lock password. Other browsers allow
selecting on inputs while user-select is none.

All z-indexes found in css files have been moved into css
variables. If the z-index is used only once, the variable
name will be the same as the selector it is used in. If
the z-index is used multiple times, then the plain name
of $zindex# was used. This allowed a more confident
moving down of the toolbar so that the new modal dialog,
with z-index 500, could display on top of it.

#1436

This reverts commit b309ef25fd.