The nodes/tags system has not been particularly successful, it's not as
intuitive as it could be, and in an attempt to be a single feature to address
many use cases, ends up addressing none of them particularly well.
This commit replaces nodes and tags with contexts.
Each job may define which context(s) the job may be associated with.
Laminar will only pop the job off the waiting queue when it can be assigned
to a context. A context defines an integer number of executors, which
represents how many runs can be simultaneously assigned to it. A context
may provide extra environment variables.
Essentially, a context replaces a node, and tags are gone. You just assign
jobs to contexts directly, and you can use a glob expression. This should be
more intuitive.
For grouping jobs in the WebUI, a separate mechanism called "groups" is provided.
