Block subscriptions + Gifting

(Don't merge) See https://github.com/HabitRPG/habitrpg-shared/pull/376

### Block & Consecutive Subscriptions

* Can subscribe to recurring monthly blocks (1m, 3m, 6m, 12m). 12m sub gives a 20% discount ($48)
* Consecutive-month subscriptions have perks: each 3m consecutive gives you +1 Mystic Hourglass, +5 gem-cap (to a gem-cap max of 50). Consecutive months can be bought in block; ie, buy a 6m sub and you get +2 Hourglasses, +10 gem-cap. Or you can just do 1m recurring till you hit that point.
* Mystic Hourglasses are used to buy old mystery-item sets. 1 MH per 1 full set. New "Time Travelers" tab for this.
  ![time travelers](http://i.gyazo.com/e24de99a2c253cc6bfc6b3e470be0c1c.png)
  If none of the old mystery sets float your boat (or you have them all), two new mystery sets (only available via Time Travelers) are available: steam punk 1 & 2.

### Gifting

* A button next to "Private Messages" lets you send gifts to friends
  ![habitrpg___your_life_the_role_playing_game](https://cloud.githubusercontent.com/assets/195202/5289179/f57817c6-7af8-11e4-983e-7b575482313e.png)
* This includes gems (either from balance or from paypal/stripe)
* And subscription blocks. This block is purchased as a one-time transaction, so the sender isn't charged recurring. As an aside, international players who can't subscribe can use this as a workaround by gifting themselves a one-time subscription block (which should work, since it's not treated as a Paypal recurring plan, but a single purchase).