Add an Elasticsearch Index selector in "Create New API" dialog

[newgene]

Under the "API" tab, we currently have a "Create New API" dialog to build a test API using the existing ES index:

The current ES index dropdown list populates all ES indices from all provided ES servers (defined in INDEX_CONFIG of config.py) first. We noticed it can be very slow or even crash the browser when this list is very large.

Let's implement a better interface to select the particular index (or alias) from the available ES servers. For example, we can select ES server first, and then allow to type a few words to filter the indices/aliases from the selected ES server.