Make badge, issuer, program undoably deletable.

This PR fixes #237 by adding the following:

**Badge deletion button in admin index page**

![screen shot 2013-06-19 at 9 55 48 am](https://f.cloud.github.com/assets/124687/675047/44604d90-d8e8-11e2-916f-afbfa532f727.png)

**Issuer deletion button in admin index page**

![screen shot 2013-06-19 at 9 55 55 am](https://f.cloud.github.com/assets/124687/675050/8e1181b6-d8e8-11e2-9c24-14c2a650a2ee.png)

**Program deletion button in program index page**

![screen shot 2013-06-19 at 9 56 11 am](https://f.cloud.github.com/assets/124687/675054/97daa538-d8e8-11e2-9c99-4f32698aa601.png)

When any of these are activated, a confirmation dialog appears like so:

![screen shot 2013-06-19 at 10 01 02 am](https://f.cloud.github.com/assets/124687/675061/c37f105c-d8e8-11e2-93a0-5ac224f5afdf.png)

Finally, this PR also adds a history section at the bottom of the admin index page that allows deletions to be undone:

![screen shot 2013-06-19 at 8 31 26 am](https://f.cloud.github.com/assets/124687/674563/3f8f9bf6-d8dc-11e2-8d2b-534badc03b0b.png)

Things to do:
- [x] Make badge/issuer/program models undoably deletable.
- [x] Make undoable deletion of program cascade to badges.
- [x] Make undoable deletion of issuer cascade to programs, badges.
- [x] Add UI/routes for deleting badges, w/ confirmation dialog.
- [x] Add UI/routes for deleting programs, w/ confirmation dialog.
- [x] Add UI/routes for deleting issuers, w/ confirmation dialog.
- [x] Add UI/routes for undeleting badges/issuers/programs.

In future PRs, we can add the following:

* Possibly moving the history section into its own page linked to from top-level nav, just before the "logout" button/link, depending on what @iamjessklein thinks.
* Adding a "purge" button next to "undo" in history list items which allows users to permanently, irrevocably delete an item, and/or busting a cronjob that automatically purges such items when they're more than a week or two old.
* Adding `request.flash()` messages that inform users of the deletion and have an immediate button/link to undo, a la gmail.