Custom showdown extensions should be linted

ref #4243

- Removing /lib/ from the list of things which aren't linted, as this is
  all custom code
- Fixing up the two files to pass linting, erring on the side of caution
  with disabling regexp rules vs fixing regexes!
Use flexbox for post settings cog & publish button

Closes #4208

- Removes `min-width` & `max-width` from `.publish-bar-actions`
- Uses flexbox to correctly handle dynamic size (due to font size and text content of button)
Abstracting away the deprecatedItems

Closes #4189

* moved `deprecatedItems` from within the function to the default config object
* addes tests
Merge pull request #4257 from ErisDS/codestyle

Custom showdown extensions should be linted
Merge pull request #4268 from PaulAdamDavis/fix-4208

Use flexbox for post settings cog & publish button
Accept Invitation: Generate Slug from Name

No issue, but ref #4211
Merge pull request #4271 from felixrieseberg/iss4211-emailslug

Accept Invitation: Generate Slug from Name
Merge pull request #4269 from javorszky/iss4189

Abstracting away the deprecatedItems
Ignore Sublime Text project files
Merge pull request #4274 from netputer/ignore-subl

Ignore Sublime Text project files
Rename .object-list to .user-list

No issue
References todo comment in [users.scss](https://github.com/TryGhost/Ghost/blob/91c07e37bf5f6467ff65181581e8c7621033ab31/core/client/assets/sass/layouts/users.scss#L6-L7)

- Renames anything starting with `.object-list` to `.user-list`
- Adjusts tests to match
Merge pull request #4276 from PaulAdamDavis/object-list-users

Rename .object-list to .user-list
Use DS.Model#eachAttribute in editor-base-controller

Closes #4279
Merge pull request #4281 from novaugust/iss4279

Use DS.Model#eachAttribute in editor-base-controller
Refactor helpers & tests into individual files

no issue

- Split theme helpers into individual files for each
- Do the same for tests
- Have utils to share some things between them
- Move assetHash onto config
Merge pull request #4283 from ErisDS/split-helpers

Refactor helpers & tests into individual files