[WIP] Add support for user limits

[jcrist]

Adds support for server-side limits of the following:

• Total cores per user
• Total memory per user
• Total clusters per user

From a user, hitting the limits results in:

• An informative error if creating a new cluster would exceed a limit
• A warning if scaling to n workers would exceed a limit

Fixes #25.

Still needs tests.

[jcrist]

From a admin perspective, adding limits looks like:

# Each user is limited to no more than 50 cores
c.UserLimits.max_cores = 50

# Each user is limited to no more than 100 GiB
c.UserLimits.max_memory = "100 GiB"

# Each user can have no more than 2 active clusters
c.UserLimits.max_clusters = 2

If a user tries to create a cluster, and doing so would exceed a limit, the operation errors:

In [6]: cluster = g.new_cluster()
...

ValueError: Cannot start new cluster, would exceed user limit of 5 active clusters.

If a user tries to scale a cluster to ``n` workers, and doing so would exceed a limit, the user gets the maximum number of workers, and a warning is shown:

In [10]: cluster.scale(2)
/Users/jcrist/Code/dask-gateway/dask-gateway/dask_gateway/client.py:576: UserLimitWarning: Adding 2 workers to cluster be8844f83a2b4773921ae7db8f3f3a9e would exceed user cores limit of 2.0, adding 1 workers instead.
  warnings.warn(UserLimitWarning(msg["message"]))

[jcrist]

Tests and configuration docs added, merging.