[Design update] Style guide for pgAdmin4

Hello Hackers,

We've started to work on creating a style guide for pgAdmin4.

Our goals with the style guide are to:
- have a single source of truth for visual design and workflows
- make it easier when adding or updating a section to the app to maintain
consistency

How we'll know we're successful:
- conversations about design are more high level, driven by user needs and
pains
- decreasing (or at least not adding to) current css bloat due to
templating and overwrites

Process for creating a style guide
We're going to start from the smallest building blocks of the site and use
that to define larger and larger components. Currently, we're taking
inventory of current styles in the app, including colors, typography,
buttons, dialogs etc.

Given how the front end is currently structured, we were thinking that
static documentation of styles would be a good MVP, at least until we
determine what the best documentation for developers on the community would
be. Attached is the inventory of styles as of now, including a Sketch file
(a tool for creating design mockups, also where I've been keeping track of
styles). You can also access files here on Google drive here:
https://drive.google.com/drive/folders/0B7jMyxF3NH5lb2JxREUxclp1Unc?usp=sharing

Please feel free to edit the Sketch file- my only ask is new versions hare
named accordingly.

*If you have worked on front end features in pgAdmin before, we'd like to
chat with you about your process and what it was like to implement styles. *Let
us know some times for a chat!

Thanks,
Shirley