Admin add plugin field reloads page without filtering on enter

**Steps to reproduce:**

1. Navigate to `/admin/plugins/install`
2. Type something in the filter field and hit enter.

**Expected Behavior**

The results are filtered same as would happen on keyup.

**Actual Behavior**

The page reloads and no filtering is performed.

Start at the admin/plugins/install page and inspect the filter field and its existing keyup behavior. Reproduce the issue by pressing Enter, then verify that filtering occurs without a page reload and matches the keyup results.

Written by the indexing model from the issue text.