Added download button in slide viewer [Enhancement]

[cjchirag7]

Description

Following changes have been made:

• Added download tool inside the slide viewer.
• Added download icon to README.md
• Added download icon to the bootstrap tour guide.

Motivation and Context

• This was marked as a 'to-do' in the uicallbacks.js file.

How Has This Been Tested?

• Tested in Google Chrome and Firefox

Screenshots (if appropriate):

Types of changes

**What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

**Go over all the following points, and put an x in all the boxes that apply.
**(If you're unsure about any of these, don't hesitate to ask. We're here to help!)

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[birm]

This looks reasonable for what it is, I'm just unsure if this is desired. That todo message has an interesting history, and goes back in some form for two years, I think to the UI prototype for the 3.0 redesign. (it was just an alert in 32bd2cd)
I'll approve since it's a good PR, but not merge yet since I don't know if we still want it. Pinging @nanli-emory to give that clarification :)