Hide some workflow buttons from users without update privileges

https://reciprocitylabs.atlassian.net/browse/CORE-10
https://reciprocitylabs.atlassian.net/browse/CORE-11
I think we should get rid of it.  Values for `context` should be:
* `'any'`
* a specific instance (with a `context` attribute)
* a Context instance
* an integer (assumed to be a context ID) (though this is only useful when passing context via `object_params`, really)

`context='for'` doesn't actually add any functionality, but complicates the code.