Global guild exp multiplier

[ForTheWout]

• [x] All exp earned when multiplier is active gets multiplied, eg: doing challenge that earns 10 exp while x2 multiplier is active results in 20 exp (wow right)
• [x] Multipliers stack, eg: when a x2 is active and another x2 is activated, the global multiplier will be x4
• [x] Display on join in chat if multiplier is active
• [x] Display in chat when multiplier is activated or deactivated
• [x] Display in challenge book in hover event in rewards, eg. Guild Exp: 25 x2
• [x] Automatically trigger x2 during weekends
• [x] Add one hour x2 option to webstore