Babel integration

[kdmukai]

Note: New pip requirement pip install Flask-Babel but I need info on how to generate a new requirements.txt that will include the hashes.

Overview: Babel will attempt to find a translation for a given wrapped string in the target language. If an entry is not found or is left blank, Babel will just return the English text. So even if there are large holes in the translation file for a target language, we won't have any broken functionality. The worst-case scenario is that the user gets English text, which is no worse than what they're seeing now.

See the new babel/README.md for most of the details.

Key changes:

• config.py: Defines the supported LANGUAGES.
• server.py: Defines a SpecterFlask class so we can add some locale-handling methods to the Flask app. Handles locale auto-detection. New endpoint to make explicit language selections. Initializes Babel to the app instance.
• helpers.py, devices.py: Multi-language support for BIP39 mnemonic wordlists. The library we're using already has all of the wordlists for other languages that are defined in BIP39. Wraps calls to Mnemonic so it can be initialized with the selected language.
• babel/README.md: docs to guide volunteer translators as well as explain to devs how to wrap strings in various situations and how to generate and compile the translation files.
• Tons and tons of string wrapping: Three main techniques used:

  from flask_babel import lazy_gettext as _
  before = "Hello, World!"
  after = _("Hello, World!")
  
  before = f"Hello, {user.name}!"
  after = _("Hello, {}!").format(user.name)
  
  # template code
  <div>Welcome!</div>
  <div>{{ _("Welcome!") }}</div>
  

  Note: using lazy_gettext everywhere rather than deciding between it and gettext. There are some pitfalls when using gettext that are solved by lazy_gettext and I don't see a downside to just always using lazy_gettext.
• language_select.jinja and language_js.jinja: template includes to add the language selection droplist anywhere in the UI. Currently on the main landing page and in Settings.

Misc:

• config.py: Explicitly forcing FLASK_ENV=development when using the DevelopmentConfig
• wallets.py: Two routes were checking for POST when the route decorator could just be updated to limit to that method. jsonify used in place of manually constructed json responses to be more pythonic.

This is still very much a draft as I'm rushing to get this PR out for everyone to take a look (i.e. not my ideal level of self-scrutiny). There are bound to be some sloppy text wrapping mistakes that still need to be caught.