Make "like" links one way

Make "like" links one way

- Updated dolike() function to accept a "un-" switch
- [frio] Updated doLikeAction() function to call dolike() instead of having duplicated code
- Added boolean logic (with explanatory truth table) to smartly delete existing activities in Model\Item::performActivity
- Moved verb/activity parameter handling closer to their use in Model\Item::performActivity
- Updated all references to dolike() and doLikeAction() to include the "un-" switch