Enshrine target contract for plugin installation functions

[PaulRBerg]

As discussed in #72, the proxy deployment cost has grown considerably since the plugin system was implemented in #53.

To save on deployment costs (which is critical for onboarding new users), we will move the plugin installation functions to a dedicated target contract (henceforth referred to as an "enshrined" target).

This is also a question of how many users will need the plugin. At a minimum, it should be a pretty safe bet to assume that not everyone will need it.

[PaulRBerg]

Done via:

• 4ca1c9d
• 3f5794a