Put dates without weekly meetings or office hours on website

[namurphy]

This PR adds dates when we will not hold our weekly meeting or office hours. Apart from holidays, the dates & reasons are:

• June 23: simultaneous SULI tutorial
• July 12 & 14: hack week
• July 26: @namurphy at CSSI meeting (should we hold weekly meeting anyway?)
• October 18 & 20: APS DPP meeting