'Autocorrect' for posts per page setting

See #3671
- As @JohnONolan’s suggestion, if the user enters an invalid value for
the ‘posts per page’ setting, the number autocorrects to 5, iOS
autocorrect-style
Icon wrapper around signing inputs

No issue

References https://github.com/TryGhost/Ghost-UI/commit/379a630e1bc1

- Adds `<span class="input-icon icon-mail">` around signing inputs to help match markup used elsewhere
Fix broken status code

closes #3779
- added custom error classes for OAuth errors (bug was caused by
oauth2orize error classes)
- added basic tests for `/authentication/token` endpoints
Merge pull request #3832 from sebgie/issue#3779

Fix broken status code
Merge pull request #3816 from felixrieseberg/iss3671

'Autocorrect' for posts per page setting
Ignore tables with pg_ prefix in pg getTables.

Closes #3825
- Fixes an issue where a PostgreSQL extension may create
  a public table and it gets caught up in the migration process.
Merge pull request #3837 from jaswilli/issue-3825

Ignore tables with pg_ prefix in pg getTables.
Merge pull request #3820 from PaulAdamDavis/input-icons

Icon wrapper around signing inputs
Pre & Next Links for Ghost_Head

closes #685
- Now that we have a ‘pagination’ meta object, we can implement
SEO-friendly `next` and `prev` ref links in `<head>`.
- This implementation works uniformly for anything that supports
pagination in the current schema (posts, tags, authors)
- Regex should make the implementation future-proof for additional
pagination
Merge pull request #3817 from felixrieseberg/iss685

Prev & Next Links for Ghost_Head
Extract starting functionality into an exported class

closes #3789
- Create a GhostServer class to manage state
- index.js now calls start on the exported server
- Alter tests to expect a GhostServer instance