accounts: add list of available filters, fix deletion of existing date filters

The "Filters" button gives a list of filterable fields, and buttons to
select the type, including true/false, text match, and on/before/after a
date. When clicked, the appropriate values are put in the search box and
the cursor is placed if any input is needed.

Dates and strings are also now matched correctly, and case-insensitively when
deleting a filter.

lang/admin/en-us.json

@@ -118,6 +118,7 @@
         "accessJFA": "Access jfa-go",
         "accessJFASettings": "Cannot be changed as either \"Admin Only\" or \"Allow All\" has been set in Settings > General.",
         "sortingBy": "Sorting By",
+        "filters": "Filters",
         "clickToRemoveFilter": "Click to remove this filter."
     },
     "notifications": {