Small improvements to training request workflow

[chrismedrela]

In the training requests list view:

• Display dates in "created at" column using "yyyy-mm-dd" format to make the column compact.
• Let users filter by state using multichoice widget (three checkboxes instead of one drop-down).
• Display only pending and accepted requests by default.
• Display email in submitter column.
• Display email in submitter and matched trainee columns in a new line, below full name, so that the columns don't take too much space.

Other stuff:

• Change validation rules so that a request is accepted when it's matched to a training (now it's accepted when it's matched with an account)
• Change title of training request edit view to something meaningful (from TrainingRequest object to Training request #11)

[chrismedrela]

@gvwilson, do you want me to enforce in validation rules that a pending request cannot be matched to a training and an accepted one must be matched to a training?

[gvwilson]

Pending cannot be matched to training.

Accepted doesn't necessarily have to be matched to training: the use
case is:

1. We have several training events coming up.
2. We score applications and accept those that are to be distributed
   amongst the events.
3. We then assign them to training events.

In this case, "accepted" is being used to filter/select applications.

[chrismedrela]

@gvwilson, do you want any other choices (like "all")?

I think that a drop-down is faster to use than three checkboxes. And definitively easier to implement. Is it fine?

[gvwilson]

Sure, and thanks.

[chrismedrela]

PR #1030