fix: tweak snackbars per material guidelines

[thevoiceofzeke]

MUMMNG-4155: "As a user who recently deleted a widget from my layout, I would like the opportunity to undo my action before the snackbar disappears, so that I may undo a delete if I want."

Material guidelines for snackbars & toasts

In this PR:

• Increased time before auto-hide
• Removed customizations that made snackbars look like notifications
• No longer show widget icons in snackbar message
• Snackbar always appears fixed to bottom of viewport (so it's always visible)

Demo/screenshot

---

Contributor License Agreement adherence:

• [x] This Contribution is under the terms of Individual Contributor License Agreements (and also Corporate Contributor License Agreements to the extent applicable) appearing in the Apereo CLA roster.

[thevoiceofzeke]

I haven't found any specific guidance either, but both Angular Material projects use 3 seconds as the default delay before hiding. I think we have wiggle room to set it to whatever we feel is appropriate.

[vertein]

I do like defaults. We can see how this goes. My gut says 4 seconds is probably enough, but we can probably see how this goes with actual users.