Add ability to download a database from LGTM

[jcreedcmu]

A possible next step for #357 is to let the user somehow supply an LGTM auth token ato allow downloading snapshots via https://lgtm.com/help/lgtm/api/api-v1#LGTM-API-specification-Snapshots

[aeisenberg]

Not sure if this will be helpful, but the github pull request extension handles authentication here:

https://github.com/microsoft/vscode-pull-request-github/blob/b3995a2c6ba85fa4f31e37035a7a6d4cb4f20c77/src/github/credentials.ts#L169

[aeisenberg]

Also, see microsoft/vscode#68738

[aeisenberg]

Implementing this is likely to be messy since VS Code does not expose any general authentication mechanism we can use for logging into LGTM. @sj suggested that it might be possible to make changes to LGTM so that downloading databases does not require an auth token. I will discuss with the LGTM team to see if this is possible.

[aeisenberg]

Unauthorized db downloads are available in LGTM now.

Since it looks like there is no easy way for a user to get a fully formed download URL, the extension will have to generate one. Here are the steps:

1. User invokes command "Download DB from LGTM"
2. User pastes in a project link like https://lgtm.com/projects/g/jasmine/jasmine
3. Extension extracts the final 3 path segments and issues a call to https://lgtm.com/api/v1.0/projects/g/jasmine/jasmine
4. Extension extracts the list of available from the result and presents a list of available databases to download
5. User selects language choice
6. Extension creates a complete link based on the language choice and the project ID that is retrieved from step 4
7. Extension downloads and unzips the database as in other commands.

This should be a separate command and separate button. We should possibly consider removing the old download database command from the databases view since this command is one that i expect to be much more useful.

[aeisenberg]

Fixed via #405.