Command Generate - Ensure that generated project on MongoDB use the latest `mongoose` dependency version for security reasons

[rap2hpoutre]

@arnaudbesnier I selected you as a reviewer for the following reasons:

• We discussed about this on slack.
• You made this commit: https://github.com/ForestAdmin/forest-express-mongoose/commit/9ee264ef8e1b6ae4103311d7b06b68581558535a.

Three things as been done (because it did not work without that):

• Since the linter do not accept template-like file, I had to rename templates/app/models/index.js to templates/app/models/index.txt (the commit can not pass without that). On a side note, the template system is currently messy: a mix of txt files, js files with invalid javascript (like this one) and handlebars code. We should improve that.
• I had to add useUnifiedTopology: true in connect. Without this, there is a warning: https://mongoosejs.com/docs/deprecations.html#useunifiedtopology (we do not use any of the option that can generate a problem: autoReconnect, reconnectTries, reconnectInterval.
• The change itself: upgrading to 5.8.2

[rap2hpoutre]

Update mongoose in template to avoid security error message

[arnaudbesnier]

To Rebase to remove the CHANGELOG.md conflict.