Bug 950242 - Admin management of group membership

Allow Django admins to manage group members as inlines on the group's
admin page.

This will work fine for reasonable-sized groups, but will get unwieldy
when groups get really large.  We might be better off managing group
membership from a user's page, since a user is unlikely to belong to
so many groups that that would get too big. Unfortunately, Django currently
doesn't support nested inlines in the admin, which would be required
because we already implement the user admin by making the userprofile
an inline on the user page, and the membership would have to be an
inline on the userprofile.

A better way for admins to manage memberships might be directly in
the GroupMembership model. To make that easier, I've added a search
field to that admin changelist page that searches on a bunch of fields
in the groups and users associated with the membership records. It
shouldn't be hard to find a user's memberships or membership in a
group, or just add a new membership.

Fixes bug 950242
Bug 959131 - Make unidecode a git submodule
Merge pull request #762 from caktus/submodule-for-unidecode

Submodule for unidecode
Update documentation regarding the version of elasticsearch.
Merge pull request #769 from akatsoulas/upate-doc

Update documentation regarding the version of elasticsearch.
[fix bug 957995] Don't autofocus on search box.
Merge pull request #759 from glogiotatidis/957995

[fix bug 957995] Don't autofocus on search box.
[Bug 942422] Add BitBucket to list of available external accounts
Merge pull request #770 from mimaison/bitbucket

[Bug 942422] Add BitBucket to list of available external accounts
Merge pull request #764 from caktus/950242-admin-group-members

Bug 950242 - Admin management of group membership
[fix bug 950242] Include 'username' in VouchedUserProfileAutocomplete.
961073 Don't link to groups page for unvouched user when editing profile

Fix bug 961073
950242 - Provide autocomplete for group names in GroupMembership admin
Merge pull request #767 from caktus/961073-only-vouched-can-join-groups

961073 Don't link to groups page for unvouched user when editing profile
Merge pull request #772 from caktus/950242-groupmembership-autocomplete

950242 - Provide autocomplete for group names in GroupMembership admin
Merge pull request #771 from glogiotatidis/950242

[fix bug 950242] Include 'username' in VouchedUserProfileAutocomplete.
Bug 962193 - Replace placeholder text on group creation page
Merge pull request #774 from caktus/962193-replace-placeholder-text

Bug 962193 - Replace placeholder text on group creation page