Datatables view parameters

Now that we have the Datatables view (https://github.com/ckan/ckan/pull/3444) that was informed by Boston's reqts, it'd be great if we can make additional improvements based on some feedback from end-users:

- [ ] be able to specify the number of entries interactively beyond the preset values in the picklist (with a configurable upper limit)
- [ ] be able to save datatables view options (e.g. compound column sorting, hidden col settings) and pass it as a URL parm (similar to the way FILTER parms are passed as URL parms)
- [ ] add a SHARE BUTTON which saves these settings in a shareable URL

beyond these user-facing parameters, it'd be nice too if the CKAN administrator can specify some settings in the INI file:

- [ ] values for Number of Entries picklist (default: 10, 25, 50, 100)
- [ ] upper limit of user-entered Number of Entries (default: 1000)

cc @therriault @wardi

Start by reviewing the Datatables view introduced in PR #3444 and trace how its current options and INI settings are handled. Done means supporting configurable entry limits, saved view options in URL parameters, and a share button that produces a shareable URL, while preserving the listed defaults.

Written by the indexing model from the issue text.