Constitutions: ignore routine changes to downloaded files

[adehnert]

​https://asa.mit.edu/groups/constitutions/#updated-in-place lists constitutions that have been updated in-place --- the URL stayed the same, but the content changed. In many cases, this is because the page in question displays the time the page was retrieved, the number of views, or a random value (CSRF token?). We should somehow exclude or separate the regularly-changing constitutions, so that we can see which groups have actually made changes to the constitution text.