Update BeamExplorer.htm (static page for historical blocks, SVG icons, column selection)

[dbadol]

Updated in v0.5 of BeamExplorer.htm:

• Add a static page listing special historical blocks.
• Update column selection request for block headers.
• Make all eye symbols toggle between expanded and collapsed text.
• Adjust the display and hover of the eye symbols.
• Replace Unicode symbols with SVG equivalents (to ensure same look on all platforms).
• Make SVG icons scale with text.
• Use SVG 'templates' to avoid repeting them inline.
• Keep footer in the bottom, even for pages of short length.
• Highlight rows on hover (for certain tables).
• Adapt, reset and simplify some CSS.
• Some cleaning details.