Finding several documents by their title in a single query

[archiloque]

It's an enhancement of the searching by title added in d98c1bd , it allow to specify several titles at the same time so all the corresponding documents are returned.

At first I though about adding a new parameter but at the end I noticed the tag parameter can be specified several times so I used the same approach.

Our use case is to be able to quickly download several files scattered in several documents from their titles and with this we can do it with only two calls:

1. one API calls find all the documents and their files from the document's name
2. one API call download the zipped files

As some times we deal with lots of files, it means we can do it with a synchronous call instead of doing it in an asynchronous job.

I had to introduce another thing (and I'm not too happy about it, so alternative suggestions are welcome) is to make the document search endpoint available as a POST call, because when when I search for a lot of documents at the same time the GET URI's are becoming too long and I get URI Too Long errors.

[jendib]

Great job. I'm ok with the POST endpoint, it's still better than Elasticsearch approach of having a GET endpoint with a request body.