Extract out Graphite HTTP functionality

For future awesomes!
Proxy all Graphite requests through Seyren

This allows exposure of Seyren only in protected setups. Authenticated Graphite users will only need Seyren to know the authentication rather than have every user authenticated too.
No need to expose any Graphite information in SeyrenConfig now
Merge branch 'refs/heads/charts-proxy'

Conflicts:
	seyren-core/src/main/java/com/seyren/core/service/checker/GraphiteTargetChecker.java
Reformatting
Fix an issue where a null last check date would prevent a check from being saved