Extract tables from webpage

Tables can be extracted from a webpage using `pandas.read_html`. We can create an interface 1) simple: where user can submit the link of the webpage and download extracted tables or 2) fancy: where user can submit the link of the webpage, see detected tables (on an image of the webpage?), un-select the tables they don't want and then download extracted tables.

No implementation files or tests are named. Start by reviewing the existing web interface and how it handles PDF table extraction, then define whether the simple or fancy workflow is in scope; done means a user can submit a webpage and download the selected tables using pandas.read_html.

Written by the indexing model from the issue text.