Append 'web' folder to assets path, closes #1088.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #1088
| Refs tickets  | --
| License       | MIT
| Doc PR        | --

I located the parameter in the `config.yml` file for now, because it shouldn't be changed by the user.