Boolean checkbox for whether alias is shown on toponym detail page (#94)

[blms]

In this PR

Per #94:

• Allow admins to control whether a LocationAlias appears as a "variant" on a toponym detail page, with a boolean checkbox

Notes

• It would be best if we could start with a list of existing variants that should be shown; then we could include it in the migration to set those records' show_on_page = True.
• I noticed that only placename_alias is actually shown in this list, so I tried to put the checkbox/list icon near it to indicate that it applies to only that field. Are researchers aware that this is the field being pulled from for this page? Is it an intentional decision, or should all aliases be potentially included?