Improve the definitions feature

[ncrocfer]

Some improvements have to be done:

V1

• include the pytz package in the requirements.txt
• use ISO8601 date format in the next_run key
• the pagination is not useful, remove it
• use the /api/definitions endpoint

V2

• add a UI view to display the list of workflows

V3

• allow the user to execute a workflow using the UI

[anthonyolea]

Updates regarding this issue since the merge of PR #150:

V1

• ~~include the pytz package in the requirements.txt~~ The package pytz was using to compute next_run key, see below.
• ~~use ISO8601 date format in the next_run key~~ The next_run value was unreliable, removed.
• ~~the pagination is not useful, remove it~~ Done.
• ~~use the /api/definitions endpoint~~ Done.

[ncrocfer]

Closing this issue as V2 & V3 features are now available.