Validate generate-assertion, get-assertion requests

[vteague]

After completion of #34 and #38 , implement validation / sanity checking

for generate-assertions:

• check description is IRV
• check contest name is consistent

for get-assertions:

• check contest name exists.

Consider whether other sanity or input validation checks are needed.

Implement tests.

[vteague]

Completed by #48.

[vteague]

Removing generate-and-get assertions from this card, and its title. Call this done when get-assertions and generate-assertions are done.