Add link to ExComm materials to info.html

[ethanmann]

Closes #207

A private Google folder link for ExComm should be added to secrets.py for this commit to work.

For testing purposes, you can add this block to default_secrets.py:

LINKS = {
    'ExComm_Meeting_Minutes':'https://www.google.com',
    'ExComm_Materials':'https://www.caltech.edu'
}

The is_excomm db attribute could be used to show/hide the ExComm Materials on the info page (I left a TODO for this).

[hannah-fisher]

This todo should be implemented before the excomm link is added to the website, so that the link is only accessible by excomm members. Could be accomplished as the comment suggests by showing/hiding the link based on whether the logged in user is part of excomm. Or, could create a separate url path to this page for excomm members that includes the link. Either way would be fine.