Add Transifex for Crowdsourcing Translations

[lidel]

(This is a part of Translation Project for IPFS GUIs, see ipfs/ipfs-gui#50 for more details)

This PR adds .tx/config which enables use of tx client for synchronizing locales with web interface at:

• https://www.transifex.com/ipfs/ipfs-companion/

Relevant docs:

• Installing the Transifex Client
• Understanding .tx/config file
• Manual sync via Transifex Client
  • Using Transifex with GitHub in Your Development Workflow
    • Syncing a local project to Transifex with the Transifex Client

TODO:

• fix this error in pl_PL
  • removed indirect placeholders: Transifex was stripping them down for some reason
• remove untranslated strings
• add .tx/config
• add CI script that detects master branch and $TX_API_KEY in env and sync source strings
  • no need, Transifex does this automatically: Meta: Translation Project for IPFS GUIs ipfs-gui#50 (comment)
• check if lang_map = zh_CN: zh is a correct convention for i18 in webextensions
  • asked Chinese team from Transifex for feedback
  • This PR applies changes from Meta: Translation Project for IPFS GUIs ipfs-gui#50 (comment) and fixes rendering of Chinese version.
  • turns out zh is not used in real life, and grabbing zh for CN is a little bit controversial, so we switched to using zh_CN
• update docs to point at Transifex, remove Crowdin
• message translators at Crowdin informing about move to Transifex
• delete project at crowdin
  • (will remove on 2018-09-14)

[lidel]

I had to remove indirect placeholders: Transifex was stripping them down for some reason :(
Switched to direct ones.