improve jekyll migration

[hamelsmu]

closes #1077

• remove unicode symbols from frontmatter title Hamel: I am not sure I should do this automatically as people may want to replace these characters with something else
• replace "tags" with "categories"
• make categories into yaml arrays
• replace post_url links with regular links
• insert date: into frontmatter (jekyll uses filename automatically)
• replace 'summary' with 'description'
• replace various include jekyll things

cc: @jph00 @seeM

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