Deepl glossaries

[sweidac]

This implements deepl-glossaries. That feature was requested by #807, but I implemented it different from the suggestion:

/**
 * Encapsulate glossary handling.
 *
 * Glossaries are enabled when the `Config.deeplGlossariesDir` setting is set.
 * Files in that directory (only direct children) that follow the naming scheme "{sourceLanguage}-{targetLanguage}.csv"
 *  are considered sources for the glossaries that are kept on the DeepL-servers.
 *
 * To transmit (create/update is the same here) these files, the user must execute one of
 *  - `Commands.deepl_update_glossaries`
 *  - `Commands.deepl_update_glossary`.
 *
 * After the glossaries have been created, they are automatically applied when translating a matching language-pair with deepl.
 */

This PR incorporates #1261 and hence #1262.

I didn't create tests because

1. I don't know where to start
2. The approach wasn't discussed ahead of implementation and might therefore change