Show auth state in Context repr.

[danielballan]

Against a mutli-user server:

In [1]: from tiled.client import from_profile

In [2]: c = from_profile('nsls2')

In [3]: c.context
Out[3]: <Context authenticated as 'dallan'>

In [4]: c.context.api_key = '<redacted>'

In [5]: c.context
Out[5]: <Context authenticated as 'dallan' with API key '86d32845...'>

In [6]: c.logout()

In [7]: c.context
Out[7]: <Context (unauthenticated)>

Against a single-user server:

In [1]: from tiled.client import from_profile

In [2]: c = from_profile('local', api_key='secret')

In [3]: c.context
Out[3]: <Context authenticated with API key 'secret...'>

In [4]: c.context.api_key = None

In [5]: c.context
Out[5]: <Context (unauthenticated)>

Needs unit tests

Closes #739

Checklist

• Add a Changelog entry
• Add the ticket number which this PR closes to the comment section

[danielballan]

I am not sure why the container-canary check is acting up. I re-ran and it failed a second time. I think it's unlikely to be related to this PR, but it might be detecting some upstream issue that is important.

[danielballan]

Canary failure was a fluke. Re-ran today; it's fine. Possibly an upstream issue.