ubuntu-support-status will report wrong dates for ESM packages

[Test case]
1. install update-manager-core from -proposed
2. run ubuntu-support-status
3. verify that a message is displayed directing the user to https://www.ubuntu.com/esm for information about Extended Security Maintenance
4. install ubuntu-advantage-tools
5. run sudo ubuntu-advantage enable-esm fakename:fakepass
6. ignore errors from being unable to retrieve esm sources
7. run ubuntu-support-status again
8. verify that no warning message about esm is displayed (though the support lengths listed are still incorrect for ESM-supported packages)

[Regression potential]
If someone has scripted around the output of ubuntu-support-status, the change in output format could cause a regression for them. This is unlikely to be a critical regression for a user and is outweighed by the benefit of directing users on a no-longer-security-supported system that they should upgrade or consider a paid security support option.

[SRU justification]
ubuntu-support-status in update-manager-core has reasonably accurate information about the support lifetime of packages in 12.04, but it does not know about ESM. Users of this command will be told that all of their packages are unsupported starting end of this month, regardless of whether they have ESM enabled on the system.

I think ideally we would:
- have a list available, shipped with update-manager-core, of all the packages supported in the ESM archive
- if the ESM archive is enabled on the system, use this source of information about the extended support length
- if the ESM archive is not enabled on the system, direct the users toward information about enabling ESM

Though the last part is a bit chicken and egg at this point, since the precise archive is closing so the only place we have to publish the updated update-manager-core package is in the ESM archive itself.