Implement upload/open filled template functionality

[kerberizer]

As discussed today:

1. Add a button named "OPEN A FILLED EXCEL DATA TEMPLATE", positioned below the "CREATE A NEW DRAFT" one.
2. Consider renaming the "CREATE" button to "CREATE A NEW DRAFT BLUEPRINT".
3. The "OPEN" button shall present a modal window with the UI for loading the Excel file via drag-and-droppping and the browser/OS file picker (which was already presented today).
4. Once an Excel file is loaded, the h5web UI will be presented on a full page/screen, with a button to download the NeXus file. To be clear, the expectation is that that h5web is shown immediately after loading the file, with no need to click additional "PREVIEW" or similar buttons.
5. Returning to the main page can be either by a dedicated "BACK TO" button or simply by clicking on the logo -- feel free to decide depending on what looks better in terms of layout.
6. The NEXUS related buttons below the tables are to be removed.

Here's a mockup of the buttons:

For reference, the technical details of the mockup are:

• the buttons are enclosed in a div to make them order themselves vertically;
• max-width: 250px was added to the buttons to keep them slimmer;
• white-space: break-spaces was set to allow the button text to break properly;
• margin-top: .5em was added for better positioning;
• background-color: #21a366 !important; was set as a color from https://www.color-hex.com/color-palette/88000 to hint at Excel.

To be clear, these changes were done just to achieve a quick mockup in dev tools -- there could very well be better ways to style the elements in the actual application.

FWIW, I've also experimented with rephrasing the description, but so far can't offer anything substantially better.

CC @vedina