Right now backend sends both background and font color information, refactor to send background color and background brightness level (needed to pick font color) and let UI apply font color. Needed to implement #18
A single silence can supress multiple alerts so to reduce response size and the amount of data react UI needs to track move it out of alert instance (which will deduplicate it) and on each alert only store a reference. In the API return all silences as a global dict and each alert only contains the ID of silences suppressing it.
This adds support for reading configuration from file, env support is still there and legacy env variables will still work, but flags are now following config schema, so they don't match old flags.
Having a config file allows to express more complex configuration options, which is needed for some feature requests.
Annotations are just string maps, but to render those we need a bit more information (is it a link, should it be visible by default). Store them as more complex structs internally, this also allows us to drop alert.Links since we now have enough information to tell if annotation is a link
Internal packages are supported by Go 1.5+, any package in /internal/ dir is only importable from the same repo. This will cleanup main dir a bit and provide better namespace for unsee subpackages
