Update CONTRIBUTING.md
Cache control headers & query string asset management

closes #1470
issue #1405

- added cache control middleware
- added defaults for all routes, assets, etc
- updated asset helper to add a query string with a timestamp hash to all assets
- added unit tests for asset and ghostScriptTags helpers
- added cache-control checks to route tests
Merge pull request #1798 from ErisDS/cache-control

Cache control headers & query string asset management
Move config path.update to config.load

issue #1789

- we can do this step a bit earlier and it aids with neatness
- also means for tests that config.load() is enough to get config & paths initialised
Merge pull request #1813 from ErisDS/path-change

Move config path.update to config.load
Error handling simplification & test improvements

fixes #1266

- simplifies the check for if a theme has an error template
- adds more / better tests for error handling
Fixes admin session cookie test

- Currently the test is taking the response date
which is in UTC and passes it through moment()
which by default parses input as local time.  We
then add 12 hours to this now local time
and when compared against the response set-cookie
header the time spread is wrong.

- To fix we’re parsing the response date with
moment.utc which parses the date in UTC.
Merge pull request #1816 from hswolff/fix-admin-session-test

Fixes admin session cookie test
Merge pull request #1815 from ErisDS/more-tests

Error handling simplification & test improvements
Update CONTRIBUTING.md

fixing the doc as per discussed in the comments for pr/1824
Merge pull request #1826 from demesne/patch-2

Update CONTRIBUTING.md