Webapp update

[16BitNarwhal]

following discussion of #2502

• exposed two new routes, /indexes and /indexes/{index}/status from the server
• webapp dropdown list is generated from /indexes route
• dropdowns use index corpus + method instead of indexname field now
• dropdowns have indicator for whether they're already cached
• /index/{index}/ is renamed to /indexes/{index}

[lintool]

Additional thoughts about API design: maybe the route should be catalog? and asking whether or not it's downloaded should be a param?

[lintool]

Also, can you attach here a screenshot so I have an idea of what it looks like now?

[16BitNarwhal]

Additional thoughts about API design: maybe the route should be catalog? and asking whether or not it's downloaded should be a param?

Whoops, saw this a bit late. I was thinking /index/{index}/cached to check status and /indexes to list all

[lintool]

I think this is a good time to document the API in an MD, e.g., docs/rest-api.md or something like that?

Provide examples like this? https://github.com/castorini/anserini/blob/master/docs/fatjar-regressions/fatjar-regressions-v0.36.1.md#webapp-and-rest-api