Update handling of network installation

[jeffpaul]

Is your enhancement related to a problem? Please describe.
When the plugin is installed at the network level, we currently only display settings on the individual site level. We may want to consider having the majority of the plugin settings at the network level and the Twitter handle as the main option on an individual site level.

Describe the solution you'd like

• keep Twitter handle on individual site settings page
• move remaining settings to a network settings page

Designs

TBD, depending on conversation from details above.

Describe alternatives you've considered

Keep as-is with network activated plugin having settings completely at an individual site level.

Additional context

See related conversation with @henrywright starting here: #57 (comment).

[henrywright]

In existing network installs, the plugin is configured at the single site level. This may cause a problem if this issue is implemented because existing settings at the single site level will no longer be used. Instead, the network administrator's setting at the network level will be active

To solve this, a "network_wide" option could be introduced with the default value being false. This would allow the plugin to be used in the same way it is used today in network installs. For example, settings would be applied separately by each site administrator in the network