Add a welcome page

[adriendelsalle]

Description

Add deps to jupyterlab-recents, jupyterlab-favorites and jupyterlab-tour Update yarn.lock [WIP] try to use the welcome page

ref: https://github.com/jupyterlab-contrib/jupyterlab-recents/pull/1, https://github.com/jupyterlab-contrib/jupyterlab-favorites/pull/1

[fcollonval]

@adriendelsalle would you mind opening the PRs on jupyterlab-recents and jupyterlab-favorites?

[adriendelsalle]

Done, I put the links above!