Add Google Tag Manager plugin and update dependencies

[pkosiec]

Description

Changes proposed in this pull request:

• Add Google Tag Manager plugin
• Update all dependencies

Testing

Optionally: Set environmental variables:

export GOOGLE_TAG_MANAGER_CONTAINER_ID="container-id"
export GOOGLE_TAG_TRACKING_ID="tracking-id"

Run npm run build and npm run serve. See the source code - you should see your IDs (e.g. gtm-dev and gtag-dev if you didn't provide the envs).