Allow specifying "latest" and "previous" pseudoversions to the /deleteversions/ and /rollback/ actions

Example:

• /rollback/ - previous:
  • http://localhost:8080/xwiki/bin/rollback/Mail/MailConfig?form_token=xWZeWeFtxBa2aRvUJ93nMw&rev=previous&confirm=1
• /deleteversions/ - previous and latest
  • http://localhost:8080/xwiki/bin/deleteversions/Mail/MailConfig?form_token=xWZeWeFtxBa2aRvUJ93nMw&rev1=latest&rev2=latest
  • http://localhost:8080/xwiki/bin/deleteversions/Mail/MailConfig?form_token=xWZeWeFtxBa2aRvUJ93nMw&rev1=previous&rev2=previous

This is very useful when the exact version number is not known and it would be, perhaps, too costly to obtain in order to use these actions. An example where this is applicable are the functional tests when we want to do basic operations as quickly and costless as possible.