Remove API url columns (that can be recalculated) from the contributors table

[MoralCode]

Note

This PR was ported from augurlabs/augur#3747 filed by @MoralCode because the contribution was deemed to still be useful.

Description
this change removes 9 columns of API urls from the contributors table, including any data in instances (because this data is entirely determined by the repo path and can be recalculated if needed

I strongly suspect these urls are unused because many of them use an uncommon pattern to indicate what appears to be substitutions (i.e. some of them contain bits like users/jimmy{/subscriptions} which would require specialized logic in order to actually use.

These URLs came directly from the github API and are inserted into the DB when we perform contributor resolution.

The main benefit is that this helps save augur instances some space in one of the most commonly used tables

This is not an especially important/urgent PR, but I figured id file it in case other maintainers have time to review it.

NOTE: this pr does not remove any of the functionality in 8knot that inserts these URLs, so this PR shouldn't merge until after this is addressed (planned in the contributor resolution updates coming soon)

Risk
The risk of causing issues with this is:

• zero, if the assumption that these urls are not used holds true

Signed commits

• Yes, I signed my commits.

[MoralCode]

• the migration number needs advancing
• the part of collection that writes to these url fields needs removing/altering
• needs check for any legacy variable/project names

This is still very much a WIP change, just filing it here to get it organized better