Require strict mode.

[NodeGuy]

• Convert to strict mode (through app/src). #1182
• Convert to strict mode (through test/unit/specs). #1183
• Convert to strict mode (through test/unit/specs/components/common). #1184
• Convert to strict mode (through test/unit/specs/store). #1185
• Use ESLint to enforce this: https://eslint.org/docs/rules/strict

[fedekunze]

@NodeGuy does this just require to add "use strict"; to the beginning of each file ?

[fedekunze]

@jbibla do you remember why do we need to split this issue ?

[NodeGuy]

We need to split this issue because if we make strict mode mandatory for all files then we're going to get a ton of non-compliance issues that need to be fixed all at once. Today I'm going to compile a list of all of the JavaScript files and create new issues to tackle groups of them at a time. Once all of the files are in compliance then we can make the ESLint change.

[faboweb]

I consider this ticket done in this sprint. The scope was to split it.

[NodeGuy]

Closing because Babel is already enabling strict mode for us with the es2015 preset. We should open this again in the future if we stop using Babel (or that preset).