Change "plugin author" to "project author" in the PTE confirmation email

[petya]

In the confirmation email that new PTEs receive after being added, there's a line that goes like this:

"You have been added to these projects either by your own request, or by the request of the plugin author."

Could we change it to:

"You have been added to these projects either by your own request or by the request of the project author."

What I did is exchange "plugin author" with "project author" since we have different types of translation projects - plugins, themes, meta projects, etc.

I also removed an unnecessary comma after "your own request".

[ocean90]

In 7050:

Rosetta Roles: Replace "plugin author" with "project author".

Props petya.
Fixes #3532.