Train dynamic models, collect results

[abenton]

• [x] Train models to predict % follower change on all data
• [x] Collect trained on all data coefficients and p-values into table 3
• [x] Train separate models on those with <152 followers, those with >739
• [x] Similarly build table 3 for each group separately (spot strategies that work well for one group and not the other). Pick history=14, horizon=28.
• [x] For selection of strategies, plot coefficient (p-value) as function of horizon
• [ ] '' '' and history