Request to join group

This pull request addresses #458
You cannot really call the template helper from outside. But you can move this function to Group document as a method and then use it both here and wherever you want. I am not sure if we have such an example already, check around. I am not sure if we should make it like:

```
Template.groupMembersList.canModifyMembership = ->
 @canModifyMembership Meteor.person @constructor.modifyMembershipPersonFields()
```

Or:

```
Template.groupMembersList.canModifyMembership = ->
 @canCurrentUserModifyMembership()
```

Or something like this. Look if we have already things like this and propose something. Or maybe this could be the difference between "has", which takes a person object, and "can" which always check for the current user:

```
Template.groupMembersList.canModifyMembership = ->
 @canModifyMembership()
```

Which you can don't even need the helper anymore, and could then simply do in the template as:

```
{{group.canModifyMembership}}
```